1. Industrial Field of the Invention
This invention relates to a so called method of detecting a light receiving position and a device for realizing the method which receives laser beams which have been projected and detects the positions of the projected laser beams when levelling or other surveys are conducted with the laser beams.
2. Related Art
There is a known art in which levelling or horizontal measuring is conducted utilizing the linear propagation characteristic of laser beams.
The above-described art is exemplified by a method, for example, as shown in FIG. 6, in which a laser beam 13 is applied to a light-receiving plate 12 which is disposed at a certain distance from a rotational laser oscillator 11 so that the level difference between the laser oscillator 11 and the light-receiving plate 12 is measured. It is further exemplified by an art in which a reference line is disposed and measuring depending upon this reference line is conducted.
FIG. 7 shows a light position detector which is formed in such a manner that a light-receiving plate 12a is divided into four pieces in X and Y directions, this light position detector representing the structure of laser beam position detection for use in, for example, automatic level detectors for motor graders or bulldozers which finish roads or runways.
A conventional laser beam receiving plate or a plate which is irradiated with the laser beams in general includes, as a photosensor, a semiconductor diode that is a light detecting device called a photodiode for converting the intensity of the received laser beam into an electric signal.
FIG. 8 shows an example of a light receiving sensor called a separation type for use in one-dimensional measurement such as levelling.
Such a separation type of light receiving sensor is provided with five photodiodes at vertical intervals in the order of millimeters, the photodiodes each having a light receiving surface 12a on the surface thereof.
As described above, in the conventional light receiving plate or the plate irradiated with light, when it is used for one-dimensional measurement, and large displacements in the span of measurement are intended to be measured, the number of the light detecting devices to be disposed needs to necessarily be increased.
For example, when the position of a laser beam applied at a certain position on an article of one meter in length is intended to be measured in precision of a millimeter unit, a light receiving plate is needed to be formed in such a manner that a light detecting device divided into 1,000 pieces is mounted in case of the conventional separate type of light receiving sensor. Namely, very large number of light detecting devices are required.
As an alternative to the above-described separation type of light receiving sensor, there is a non-separation type of light receiving sensor in which the difference in resistance is detected depending upon the incident position of a laser beam. However, since it involves unevenness of intensity and spread of the luminous point of laser beams, the calibration becomes complicated. Therefore it is not suitable for use as a measuring instrument. Furthermore, the accompanying processing circuit becomes complicated, and comprises an analog circuit. Therefore a problem arises in that sufficient precision cannot be obtained.
Furthermore, the size of one light receiving sensor reaches several centimeters. Therefore in case of measurement in an elongated span, a multiplicity of the non-separation type light receiving sensors needs to be connected for use, causing the cost of the device to become very large.
As described above, in case a large one-dimensional measuring span is required, a problem arises in that a multiplicity of very expensive light receiving sensors are needed to be used.
As an alternative to the above-described means, a method for use in two-dimensional measurement is known in which an image obtained by a CCD or image pickup tube is processed so that the incident position of a laser beam is detected. However, when the number of pixels are increased for the purpose of improving precision, several tens of seconds to several minutes are taken for processing one frame. Therefore it is not suitable for use in high speed processing, and is impractical.
In any of the above-described methods, in case where the region to be measured is widened in one or two-dimensional manners, the number of requisite light detecting devices becomes large. As a result of this, problems arise in that light receiving plate or plate irradiated with light have an exceedingly high cost, and also its processing circuit becomes complicated.
Therefore a group including the inventor of the present invention found a method in which the number of photodiodes is decreased by way of connecting light-branching device comprising a plurality of output ports (see U.S. Application No. 07/207,627 filed June 16, 1988 Japanese Patent Publication No. 082442/1987).
FIG. 9 illustrates the basic structure of the method in which reference numerals 1 to 16 represent optical fibers, reference numeral 17 represents a light receiving surface formed by the edges of the optical fibers 1 to 16, and reference numerals 1A to 16A represent light branching devices connected to the corresponding optical fibers 1 to 16. Reference numerals 1a, 1b to 16a and 16b represent output ports of the corresponding light branching devices 1A to 16A. Symbols A, B, C and D represent light receiving sensors.
However, the method in which the light branching devices 1A to 16A are employed involves a problem in uniformly branching the light. Furthermore the cost of the light branching devices 1A to 16A is relatively high. Therefore a problem arises in that the total cost of the device cannot be reduced although the number of the photodiodes can be decreased.
Therefore a group including the inventor of the present invention studied for the purpose of achieving a method of and device for detecting the light receiving position wit low cost which can overcome the above problems experienced, and in which the measured range is wide, and precise and high speed detection of the laser beam receiving positions can be conducted even if the area to be measured is wide.
As a result of this, the following method of and device for detecting light receiving positions can be obtained.